Low noise electron gun



1965 J. BERGHAMMER ETAL 3,226,595

LOW NOISE ELECTRON GUN Filed March 31, 1960 INVENTORS Ja /{Awayit'id/MMMEA 74/VZE) 5100M United States Patent 3,226,595 LOW NOISEELECTRON GUN Johannes Berghammer, Princeton, and Stanley Bloom,Plainfield, N.J., assignors to Radio Corporation of America, acorporation of Delaware Filed Mar. 31, 1960, Ser. No. 19,079 1 Claim.(Cl. 315-16) The present invention relates to an apparatus and methodfor producing a low noise electron beam from a thermionic cathode. Theinvention is useful in any electron beam tube requiring a low noisebeam, such as traveling wave tubes and beam klystrons.

The electron stream or beam emitted by a thermionic cathode emerges withrandom noise in the form of current and velocity fluctuations due to therandom emission of electrons from the cathode and to their thermalvelocity spread, and other causes. This noise consists of an infinitenumber of waves at different frequencies. When the beam is used in anamplifier tube for amplifying signals in a given operating frequencyrange, the noise waves having frequencies in that range are amplifiedalong with the signal and result in an undesirably low signal-tonoiseratio, or noise factor. Thus, it is desirable to reduce or minimize theinitial noise in the beam in the operating frequency range as much aspossble.

Most so-called low noise tubes now in use merely transform the initialnoise in the beam in the gun region so that the beam enters the helix orother R.F. interaction region in a favorable condition, without actuallyreducing the total noise content of the beam appreciably.

The object of the present invention is to produce a low noise electrongun that will reduce or attenuate the total noise in the beam from athermionic cathode to a very low value.

A further object is to combine such a noise reducing gun region with asubsequent exponential transformer region ahead of the RIF. interactionregion of a traveling wave tube, for example, to obtain a minimumoverall noise factor for the tube.

These and other objects .are attained, in accordance with the presentinvention, in an electron gun comprising a thermionic cathode and atleast two annular electrodes spaced substantially along the beam pathfrom the cathode by applying suitable low positive voltages to the twoelectrodes relative to the cathode to produce a low velocity beam havinga virtual cathode in the region between the two electrodes and having aplasma frequency in the virtual cathode region substantially greaterthan the desired operating frequency.

In the accompanying drawing:

FIG. '1 is an axial sectional view of the electron gun portion of anelectron beam tube embodying the present invention;

FIG. 2 is an enlarged axial, sectional view of a modified cathode thatmay be used in FIG. 1;

FIG. 3 is a graph used in explaining the invention, and

FIG. 4 is an enlarged axial sectional view of an alternative cathode andassociated accelerating electrode that may be used instead of thecathodes of FIGS. 1 and 2.

In FIG. 1 is shown a beam tube having an envelope 1 containing anelectron gun structure 3 comprising a thermionic cathode 5 having anelectron emissive face 7 on one end and a recess 9 at the other end forreceiving a conventional heater (not shown). Preferably, the cathode 5is cylindrical with a circular end face 7 coaxial and normal to thelongitudinal axis of the envelope 1. The cathode 5 may be formed with arelatively narrow peripheral portion of the end face 7 sloped away fromthe central portion of the end face 7 and provided with an annularemissive coating 1'1, as shown in FIG. 2. The narrow coated sloped edgereduces erratic edge emission of electrons that contributes toward thenoise in a beam from a cathode having a square edge as shown in FIG. 1,limits the emission to the outer portion of the cathode to produce auniform hollow beam 13 along the axis of the tube, and increases thelifetime of the cathode. Moreover, the sloped surface permits theequipotential surfaces to form more smoothly around the emittingsurface.

The beam 13 emitted by the cathode 5 is accelerated by relatively smallpositive voltages applied to two aligned afinular acceleratingelectrodes 15 and 17 spaced substantially along the beam path from thecathode and spaced apart by a distance d, as shown in FIG. 1. An exampleof a suitable potential distribution along the beam edge between thecathode 5 and electrode 17 is shown by the curve 19 at the top ofFIG, 1. Preferably, the positive voltage applied to electrode 15 is notgreater than about 10 volts and that applied to electrode 17 is fromabout 3 volts to about 30 volts, in order to keep the average beamvelocity v to a relatively small value.

To produce the desired virtual cathode in the space between electrodes15 and 17, and also to make the plasma frequency, w in the virtualcathode substantially greater than the operating frequency, twothoustand megacycles, for example, requires a relatively large beamcurrent density, of the order of ma./cm. In order to assist theaccelerating electrode 15 in drawing out the required beam current, anadditional annular accelerating electrode 21 is coaxially positionedaround the cathode 5. This electrode is preferably of truncated-coneshape with its inner edge positioned behind the end face 7, as shown inFIG. 1, so that it can be operated at a small positive voltage ofseveral volts without intercepting electrons of the beam. The field ofelectrode 21 fringes around in front of the end face 7 and therebyincreases the beam accelerating electric field gradient.

The electrons in the beam 13 are confined or constrained to traversepaths parallel to the axis of the tube by an axial magnetic field, suchas that produced by a coil 23 coaxially surrounding the envelope 1.

Preferably, separate adjustable voltage sources 25, 27 and 29 are usedto apply the desired positive D.C. voltages to electrodes 15, 17 and 21respectively, relative to the cathode 5, as shown schematically in thedrawing.

In the operation of the tube, the cathode is heated by conventionalmeans to thermionic emitting temperature and the voltage sources 25, 27and 29 are adjusted, for the particular geometry used, to cause avirtual cathode to be formed in the beam path between electrodes 15 and17. The virtual cathode is indicated by the dip of the voltagedistribution curve 19 slightly below the base line, which is the Zero orcathode potential. It will be shown how the electron gas or plasmaconstituting such a virtual cathode can be caused to support onlyevanescent waves, i.e., perturbations that decay in the direction awayfrom the source, and hence, reduce the total noise in the electron beam.

First, consider a stationary (non-drifting) electron gas without thermalor other motions. The oscillation frequency, w, of the electron gas isthen just the plasma frequency, w Thus we write Next, suppose that avelocity spread exists in the electron gas in a confining magnetic fieldin the z direction. Let 6 represent the velocity spread; +6/2 is thehighest velocity, and -6/2 is the lowest velocity. Then, the dispersionEquation 1 becomes Solving this equation for [3, we obtain twosolutions, as follows:

(10/110 (.0 /U (.0 a1.2= ,1 /1a (1 where the subscripts l and 2 for #2represent the two roots (or waves) for the plus and minus signs, respectively, and

is the relative velocity spread. In terms of beam temperature a =3kT /mvwhere k is Boltzmans constant and T is the beam temperature in degreesKelvin. Equation 4 may be written fiLz fio flp where and The term [3,,is the plasma propagation constant. Examination of Equation 5 shows thatB is imaginary when Fig 6) in which case [3 (Equation 4) is complex. Itis known that if the propagation constant of a medium is complex, wavespropagated by that medium must be either amplified or reactivelyattenuated by the medium. By using criteria given in a paper entitledKinamatics of Growing Waves, by P. A. Sturrock, Physical Review, vol.112, No. 5, pp. 1488-1503, December 1, 1958, it can be shown fromdispersion Equation 3 that for these waves there is no growth with timepossible, hence, these waves are evanescent.

Thus it can be seen that the total noise fluctuations in an electronbeam can be reduced by establishing conditions in the beam pathsatisfying condition 6.

FIG. 3 shows the curve 31 for the equation The shaded area above thiscurve represents condition 6. It can be seen that condition 6 includesthe following which means that some electrons must have negativevelocities. This, in turn, means that a virtual cathode must exist inthe electron stream to satisfy conditions 6 and 8. Condition 9 says thatthe plasma frequency cu (Bi /7716 11 .where e/ m is the charge to massratio of the electron,

i is the beam current density, and e is the dielectric constant of avacuum, this requires a relatively high D.C. current density i,,. If,for example, the virtual cathode is produced under conditions in which ais not much greater than unity, it is necessary that u be substantiallygreater than 0:, in order to satisfy condition 6. Conversely, if u isnot much greater than w, a must be substantially greater than unity.

The electron gun shown in FIG. 1 is designed to produce both a virtualcathode and a large D.C. current density. The gun is operated, inaccordance with the present invention, with applied voltages such that(l) a virtual cathode is formed in the beam path between electrodes 15and 17, and (2) the plasma frequency in the virtual cathode issubstantially greater than the operating frequency of the tube.

In order to produce a virtual cathode in a beam be-- tween two spacedelectrodes, the beam current density must exceed a certain criticalvalue, given, for example, on page of an article entitled Effects ofSpace Charge in the Grid-Anode Region of Vacuum Tubes, by Salzberg andHaeif, RCA Review, January 1938, as follows:

ara' m in c.g.s. units. Converted to practical units and presentnomenclature, Equation 12 becomes where V and V are the positivevoltages applied to electrodes 15 and 17, respectively, and d is thedistance between these two electrodes. Thus, for a particular set ofvalues for V V and d, it is necessary to inject a beam current density Iwhich is somewhat greater than I from Equation 13. The combinedaccelerating field produced by electrodes 15 and 21 is sufiicient toinject the current density required to produce the desired virtualcathode between electrodes 15 and 17 and make w substantially greaterthan w.

The tube shown in FIG. 1 further includes three additional annularaccelerating electrodes 33, and 37 spaced along the beam path beyondelectrode 17. The last accelerating electrode 37 includes a drift tubeportion 39 to which a helix 41 is connected by an RF. coupling portion43 adapted to be coupled to an external transmission line. Preferablyelectrodes 33 and 35 are positioned so that the distance between thevirtual cathode (between electrodes 15 and 17) and electrode 33 is lessthan the spacing between electrodes 33 and 35, as shown, and suitablevoltages are applied to electrodes 33, 35 and 37 by a voltage source 45to produce an exponential rise in space potential from electrode 17 toelectrode 37. As an example, the voltages applied to the variouselectrodes may be as follows:

Electrode: Volts 5 0 21 5 15 3 17 3 33 5O 35 37 250 Preliminaryexperiments have been made with a low noise traveling Wave tube havinggun electrodes similar to those shown in FIG. 1 but without the slopededge on the cathode, normally operated as an exponential transformertype gun to give an overall noise factor of 56 db. When this tube wasoperated, in accordance with the present invention, with suitablevoltages on electrodes 15, 5, 17 and 21 to produce a virtual cathode inthe beam between electrodes 15 and 17 an overall noise factor of about 3db was obtained at a frequency of 3000 megacycles. It is believed thatnoise factors as low as 1 db can be obtained by optimum electrode designand operating voltages.

FIG. 4 shows an alternative cathode 47 in the form of a hollow cylinderhaving an annular fiat end face 49 coated with emissive material. Thecathode 47 is coaxially surrounded by an annular electrode 51, likeelectrode 21 of FIG. 1. An additional accelerating electrode 53 in theform of a cylindrical rod is coaxially mounted within the hollow cathode47 with a conical end portion protruding somewhat beyond the end face49. In operation, low positive voltages are applied to electrodes 51 and53, relative to the cathode 47, to augment the accelerating field ofelectrode 15. If desired the inner and/or outer edges of the cathode endface 49 may be sloped like the cathode face 7 in FIG. 1, for the samepurpose.

What is claimed is:

An electron gun for producing a low noise electron beam along apredetermined axis, for use in a given operating frequency range,comprising a cylindrical thermionic cathode having a circular electronemissive end surface coaxial with and normal to said axis, theperipheral portion of said end surface being sloped backward and outwardfrom the central portion thereof and coated with emissive material, twoannular accelerating electrodes coaxially disposed along said axis inspaced relation to each other and spaced substantially from saidcathode, and means including at least one voltage source connectedbetween said cathode and said accelerating electrodes for applying suchlow positive voltages to said electrodes relative to said cathode as toproduce a low-velocity confined beam having a virtual cathode in theregion between said accelerating electrodes and having a relativevelocity spread in said virtual cathode, where w is any operatingfrequency in said range, to is the plasma frequency and w is greaterthan (0, said means including a third annular accelerating electrodecoaxially surrounding said thermionic cathode, with the cathodeprotruding into the space between said first and third acceleratingelectrodes.

References Cited by the Examiner RCA Review; June, 1960; page 244 cited.

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ROBERT SEGAL, Examiners.

